Hi aspstarter,
You can retrive like below.
List<string> productId = new List<string> { "11", "12", "13" };
var product_Id = from id in productId select id;
Session["Product_ID"] = product_Id;
//Add id to product list.
productId.Add("14");
//Get the product list in other page.
var result = Session["Product_ID"];
List<string> product_ID = (result as IEnumerable<string>).ToList();
string product_ID1 = product_ID[0];
string product_ID2 = product_ID[1];
string product_ID3 = product_ID[2];
string product_ID4 = product_ID[3];